In this April 9, 2015 file photo, former Maryland Gov. Martin O'Malley greets local residents before speaking at a fundraiser in Indianola, Iowa. AP Photo/Charlie Neibergall WASHINGTON (AP) — Former Maryland Gov. Martin O'Malley is ready to enter the Democratic presidential campaign, mounting a longshot challenge to Hillary Rodham Clinton for the party's nomination in 2016.

O'Malley plans to launch his campaign on Saturday in Baltimore, where he served as mayor before winning two terms as governor. He was set to fly to Iowa for appearances in the early voting state later that day.

The former governor has made frequent visits to Iowa and New Hampshire in recent months, but remains largely unknown in a field dominated by Clinton. He'll also be competing with Vermont Sen. Bernie Sanders, in what could become a tussle for the support of the Democratic left.

O'Malley met Democratic donors in his hometown on Friday night and has offered not-so-subtle hints about his decision. His advisers released a YouTube video of O'Malley strumming "Hail to the Chief" on a guitar. The video shows him nodding his head in agreement, followed by the words, "Stay tuned." In his spare time, O'Malley fronts an Irish rock band called O'Malley's March.

An ally of former President Bill Clinton, O'Malley was the second governor to endorse Hillary Clinton's presidential campaign in 2007. But he has said Democrats deserve a choice in the presidential primaries.

In events leading up to his announcement, the 52-year-old O'Malley spoke often about the economic challenges facing the nation and has said he would bring new leadership, progressive values and a record of getting things done as governor to the White House.

O'Malley has presented himself to voters as a next-generation leader for the party, pointing to his record as governor on issues such as gay marriage, immigration, economic issues and the death penalty.

His announcement is coming weeks after riots in Baltimore that broke out following the death of Freddie Gray, an African-American man who died in police custody following his arrest last month.

Demonstrators planned to gather near the city's Federal Hill Park during O'Malley's announcement to protest his criminal justice policies as mayor, an office he held from 1999 until his election as governor in 2006. O'Malley was known for his tough-on-crime, "zero tolerance" policies that led to large numbers of arrests for minor offenses; critics say it sowed distrust between police and the city's black community but supporters note the overall decrease in violent crime during his tenure.

Maryland Governor Martin O'Malley speaks at a campaign rally for Lieutenant Governor Anthony Brown, Democratic nominee for Maryland governor, at the University of Maryland in College Park, Maryland October 30, 2014. Reuters

O'Malley has defended his work to curb crime in the city, saying he helped address rampant violence and drug abuse. He has said the unrest in Baltimore should wake up the nation to the need to address despair in poor communities.

O'Malley could soon be joined in the Democratic field by former Rhode Island Gov. Lincoln Chafee, who plans to make an announcement next week, and former Virginia Sen. Jim Webb, who is exploring a potential campaign. Sanders has raised more than $4 million since opening his campaign in late April and sought to build support among liberals in the party who are disillusioned with Clinton.

One of O'Malley's first tasks as a candidate would be to consolidate support among Democrats who are reluctant to back Clinton and eyeing Sanders.

"His first real hurdle here is not Secretary Clinton, it's Senator Sanders," said Craig Varoga, who was O'Malley's chief strategist during his 2010 re-election campaign but is not currently advising him. "There's no ambiguity at all with Senator Sanders on the issues and he came out of the gate with strong fundraising. He set a pretty high bar."

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